In general, radio transceivers operate in environments where interference is present. In addition to desired signals, noise and unwanted signal sources cause interference. For example, a radio transceiver may receive strong unwanted signals out of the desired signal range. These so called blocker signals must be filtered out from the received signal early in the receiver path of the transceiver. Preferably, the blocker signals should be filtered out from the received signal before analogue-to-digital conversion in order to prevent intermodulation distortion and aliasing.
Continuous-time delta-sigma or ΔΣ modulators may be used as the A/D converter of a receiver. The use of ΔΣ modulators relax the antialias filtering requirement but even with these modulators strong out-of-band signals may render the modulator unstable or at least create distortion.
Document K. Phillips et al, “A continuous-time ΔΣ modulator with increased immunity to interferers”, IEEE J. of Solid-State Circuits, vol. 39, December 2004, discloses a known counter-measure against blocker signals. In the solution, the feedback signal of the ΔΣ modulator is predistorted with an analogue filter. This method is illustrated in FIG. 1. The ΔΣ-modulator comprises a forward path 100, a comparator 102, and a feedback loop 104 comprising a D/A converter 106 and an analogue high pass filter 108. Unfortunately, this method does not decrease the circuit complexity, since in order to save one filter stage in a prefilter before an A/D converter, two filter stages have to be added to the A/D converter. Additionally, the high pass filtering of the D/A converter output easily requires operational amplifiers with higher slew rate than other operational amplifiers in the modulator, thus further increasing the power consumption.